The present invention relates to a combined implantable defibrillator-pacer cardiac treatment system utilizing a pacer tip lead switch for the protection of the electronics during high-energy defibrillation.
One arrangement of a combined implantable defibrillator-pacer includes a bipolar pacing tip mounted on the heart and two patch electrodes mounted on or about the heart for defibrillating the heart. The two patch electrodes may be designated a defibrillation patch and a common patch. In such an arrangement the pace output conduction path is from the bipolar pacing tip to the common patch electrode. The defibrillation conduction path is from the defibrillation patch electrode to the common patch electrode. One problem encountered by this configuration is that during high voltage defibrillation, the pacing tip electrode acts as a probe and conveys the high voltage delivered during defibrillation to the input of the implantable device. This high voltage would normally destroy the implantable device. One way to prevent this from occurring is to place a diode, with its anode connected to the pacing tip and its cathode connected to common, between the pacing tip electrode and the common return path. This diode clamps the high potential seen at the pacing tip and protects the device from the high voltage during defibrillation. However, this diode creates a further problem.
By placing the diode between the pacing tip and the common return path, a current conduction path is treated from the high voltage defibrillation patch electrode, through the heart, through the pacing tip electrode, and through the diode to the common return path. This current can be as high as 4 amps, which is sufficient to burn myocardial tissue at the pacing tip-electrode interface, as well as increase pacing thresholds.
One solution is to open this current conduction path by placing a large resistance in series with the pacing tip lead to limit the current therethrough. However, since the implantable unit is also used as pacemaker, such a large resistance attenuates the pacing pulse amplitude to such a low level that pacing the heart becomes practically impossible without dramatically increasing input energy requirements.